1. Field of the Invention
The present invention relates to an image taking apparatus capable of preventing flickers from being generated in a video signal, a correction circuit and a correction method.
2. Description of the Related Art
In the past, the image-quality performance of a CMOS (Complementary Metal Oxide Semiconductor) device was not more excellent than the image-quality performance of a CCD (Charge Coupled Device Image Sensor) device. In recent years, however, the image-quality producing performance of a CMOS device is comparable with the image-quality producing performance of a CCD. For this reason, the CMOS device is used mainly in the video field recently.
If an image taking device such as a CMOS device with an exposure timing varying from horizontal line to horizontal line is used by applying conditions determined in advance, a strip pattern with standstill contrast is generated in the horizontal direction of the taken image. The strip pattern with such contrast is attributed to the blinking period of the light source and is referred to as a standstill flicker. In the following description, the standstill flicker is referred to merely as a flicker. It is to be noted that a contrast stripe pattern generated in the horizontal direction is referred to as a moving flicker. Conditions for generation of a flicker are listed as conditions (1) to (3) which are described as follows:
(1): An image is taken by making use of a blinking light source such as a fluorescent lamp.
(2): The blinking period of the light source is equal to the integral multiple of image taking frames taken per second.
(3): An electronic shutter is used in the image taking operation.
If an image is taken in a state where conditions (1) to (3) are all satisfied, a flicker is generated. If only condition (2) is satisfied, no flicker is generated.
By referring to diagrams of FIGS. 1 to 3, causes of the generation of a flicker are explained. In this case, the light source cited in condition (1) is a fluorescent lamp driven at a power-supply frequency of 60 Hz (In the following description, the fluorescent lamp driven at a power-supply frequency of 60 Hz is also referred to simply as a 60-Hz fluorescent lamp). The number of horizontal lines of the CMOS device is 1125.
FIGS. 1A and 1B are a plurality of diagrams showing exposure timings of horizontal lines of the CMOS device used in an image taking operation which makes use of a 60-Hz fluorescent lamp. To be more specific, FIG. 1A is a diagram showing a graph representing changes of the brightness of the 60-Hz fluorescent lamp along the time axis. The horizontal axis of the diagram is the time axis. As is obvious from a waveform 102, the blinking period of the fluorescent lamp is a period which is a frequency of 120 Hz.
On the other hand, FIG. 1B is a diagram showing exposure start times. In order to satisfy condition (2) described above, the frame rate of a video taken by making use of an image taking device is 60 frames/second. That is to say, the time (exposure time) that it takes to take an image of a frame is 1/60 seconds. In addition, the shutter speed of an electronic shutter set in the CMOS device as the electronic shutter mentioned in condition (3) is equivalent to a time period of 1/2000 seconds.
If an image is taken by applying the three conditions described above, a video actually used is a video taken only during an effective exposure time 103 of 1/2000 seconds in the exposure time of 1/60 seconds for each horizontal line. That is to say, a video taken during an ineffective exposure time 104 is not used. It is to be noted that there are horizontal lines 1 to 1125 of the CMOs device and, as described above, the exposure time of 1/60 seconds for each horizontal line is the time that it takes to take an image of a frame.
In addition, the exposure processes of the horizontal lines of the CMOS device are carried out sequentially in an order starting from horizontal line 1 to horizontal line 1125. The sequential execution of the exposure processes corresponds to the procedure for taking an image of a frame. Then, as the operation to take an image of a frame is ended, horizontal line 1 is subjected to the exposure process. In order to take an image of a frame (or 1125 horizontal lines) in the exposure time of 1/60 seconds, the timing shift between the starts of the exposure processes for adjacent horizontal lines is thus ( 1/60)/1125 seconds.
As described above, if an operation to take an image is carried out by making use of a 60-Hz fluorescent lamp with the shutter speed of the electronic shutter set at 1/2000 seconds, the amount of light entering the horizontal lines of the image taken device can be computed by integrating the waveform 102 over the effective exposure times 103 of the horizontal lines. Changes of the amount of light from horizontal line to horizontal line are represented by a waveform 202 shown in the diagram of FIG. 2. The period of the waveform 202 is a frequency of 120 Hz and is thus equal to the blinking period of the fluorescent lamp.
In actuality, if an operation to take an image of a photographing subject is carried out by making use of a 60-Hz fluorescent lamp with the shutter speed of the electronic shutter set at 1/2000 seconds, light entering the horizontal lines of the CMOS device is light reflected by the subject of photographing. Thus, an image 303 shown in a diagram of FIG. 3 as the image of a frame determined in advance includes a strip pattern which exhibits standstill contrast and has a period equal to a frequency of 120 Hz. That is to say, the luminance changes of the horizontal lines of the image 303 are variations attributed to the subject of photographing as variations superposed on periodical variations caused by a flicker as shown by the waveform 302. In the case of such a standstill flicker, luminance changes due to the blinking state of the fluorescent lamp are hardly generated between pixels composing the present frame and the same pixels composing the frame immediately leading ahead of the present frame.
By the way, a representative technology for correcting a flicker is disclosed in documents such as Japanese Patent Laid-Open No. Hei 11-164192 (for example, referred to as Patent Document 1). In accordance with this technology, a flicker is controlled by adjusting the gain of every control signal by making use of a correction value which is computed as a value for controlling the gain of every control signal and the luminance. The gain of every color signal and the luminance are controlled by making use of waveforms of conversions of the luminance and the color phase which are observed on the screen. The luminance and the color phase are generated by a beat tone determined by a blinking frequency of an electrical discharging phenomenon or the like and a frame frequency or a field frequency.